A. Field of Invention
The present invention involves devices and methods related to surgical bone fixation technology.
B. Description of Related Art
Bone is a remarkable material, and rare in its potential to heal completely following significant trauma. This potential is, at least in part, a product of the continuous remodeling that living bone undergoes and may only be fully realized under conditions conducive to healing. The goals of the treatment of fractures can include reduction of the fracture to as close to normal anatomy as possible and/or fracture fixation that allows the bone to heal without complications. Better fixation typically promotes more rapid healing and thus allows earlier load bearing.
Bone generally can heal in two ways, which may be referred to as primary and secondary healing. Primary healing is typically achieved by tunneling of osteoclasts; cells specialized to resorb bone, across the line of the fracture. This is followed by growth of blood vessels down the length of the tunnel and deposition of bone filling in the tunnel walls by osteoblasts, cells specialized in depositing bone. In this fashion, new bone is produced that spans the fracture and replaces the broken bone. Secondary healing typically proceeds by the formation of a callus (e.g., a large mass of collagen and granulation tissue). During this process, there is resorption of strained areas of bone adjacent to the fracture. Woven bone is deposited in the callus in an unstructured manner and is then remodeled by osteoclasts and osteoblasts to produce the healed bone.
Primary healing is desirable, in part, due to its lower risk of complications and potential for shorter healing time. However, primary healing is typically possible only with substantial, if not absolute, stabilization of the fracture (e.g., by fixation). Proper fixation may also reduce the risk of non-union (e.g., where the fracture fails to heal and fibrous tissue is instead produced). The reasons for this largely depend on the structure of the bone and/or the way in which the bone cells respond to the loads that are applied to the bone (e.g., relative movement of bone fragments is highly undesirable).
Current technology for internal fixation of fractures includes a variety of approaches. The most common of these include screws, compression plates, medullary nails, staples, and wires. These devices may be made from a variety of materials whose properties are compared to bone in Table 1 below. Screws are usable in fractures where it is possible to drill a hole at an angle to the fracture so that the screw compresses the fracture as it is tightened. Compression plates can be affixed to a fractured bone in a way that spans the fracture and compresses two pieces of bone together (e.g., by placing the plate itself under tension). Medullary nails are rods driven into the medullary cavity of fractured long bones and can serve to align the bone fragments while only partially bearing the load. Staples are typically driven into bone so that they span a fracture and provide fixation, but may not be capable of providing significant compression. Metal wires have been used to apply initial compression to fractures by tightening the metal wire (e.g., in various lacing methods).
TABLE 1Properties of Implant Materials and BoneYieldUltimateFatigueElastic ModulusStressStressEnduranceMaterialGPaMPaMPaMPaTi—6Al—4V110800965414316L SS200700965345Co—Cr—Mo (cast)210450655310Co—Cr—Mo (forged)2108961207414Bone1713015034